Some refrigeration systems comprise two or more refrigeration entities, e.g. in the form of display cases or refrigerated rooms. Each refrigeration entity comprises one or more evaporators, each being adapted to be in an active state in which a flow of refrigerant is allowed to pass along an evaporating surface, and in an inactive state in which such a flow is prevented. The evaporators can be switched between the active and the inactive states in order to obtain and maintain a temperature within a desired temperature range inside a refrigeration compartment of the refrigeration entity. Refrigeration systems of this kind often comprise a variable capacity compressor device, e.g. in the form of a compressor rack of two or more compressors. Thereby it is possible to adjust the refrigeration capacity of the refrigeration system (i.e. the amount of liquid refrigerant removed by the compressors from the evaporators of the refrigeration entities) to meet a refrigeration demand (i.e. the amount of gaseous refrigerant produced by the evaporators). In case the variable compressor device is in the form of a compressor rack, the refrigeration capacity is typically adjusted by switching compressors of the compressor rack on or off. If this is done relatively frequently, it results in significant wear on the compressors. The refrigeration demand of each refrigeration entity depends on the applied control strategy and on the external load on the refrigeration entity in question. In the present context the term ‘load’ should be interpreted to mean the heat applied to the refrigeration entity. Thus, changes in the load are normally caused by exterior effects, such as positioning new (most likely warmer) products in a refrigeration compartment of a refrigeration entity, or applying a night cover to one or more refrigeration entities.
It is desirable to keep the total refrigeration demand of the refrigeration system as close to the total load of the refrigeration system as possible. Thereby compressors need only to be switched on or off when it is necessary to meet a change in the load caused by exterior effects as described above. Thereby switching compressors on or off is avoided to the greatest extent possible.
Refrigeration systems as the ones described above are often used in supermarkets, where several display cases are normally present.
It has previously been attempted to reduce the wear on compressors of a refrigeration system by reducing the number of times the compressors are switched on or off. One example of such an attempt is disclosed in U.S. Pat. No. 5,533,347. U.S. Pat. No. 5,533,347 discloses a method of controlling a refrigeration case of a refrigeration system. The method comprises the steps of locating an expansion valve of a refrigeration case, moving the expansion valve of the refrigeration case out of a predetermined position, sampling temperature readings from an evaporator coil of a refrigeration case, calculating an adaptive proportional-integral-differential (PID) of the temperature of the evaporator coil for the refrigeration case, and moving the expansion valve to a second position closer to the predetermined position based on the calculated proportional-integral-differential until the temperature of the evaporator coil for the refrigeration case is within a predetermined temperature range. One advantage is that the amount of time may be increased between switching on and off the next stage of the compressor rack by using the deadband area of control within each refrigeration case load to regulate only when the compressors cannot maintain control. Thereby switching compressors on or off is avoided to some extent.
However, when large variations in the refrigeration demand occur, the method disclosed in U.S. Pat. No. 5,533,347 is not sufficient to meet the refrigeration demand, and it is therefore necessary to switch compressors on or off. It is, therefore, desirable to be able to even further reduce the wear on the compressors.